1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method for improving the performance of handovers between different mobile communication systems.
2. Description of Related Art
The Global System for Mobile Communications (GSM) has been a state-of-the-art digital Time Division Multiple Access (TDMA) mobile communications system for a number of years. A successor to the GSM and other existing digital systems is the so-called 3rd generation mobile telephony system or Universal Mobile Telecommunication System (UMTS) currently under development. In order to offer global coverage for UMTS end users when this new system is first introduced, it will be possible to perform handovers between UMTS and GSM networks. In this way, the GSM networks will be able to provide coverage in those regions where UMTS network coverage is not provided. Consequently, when a mobile station (MS) sets up a call in a region where there is no coverage provided for UMTS users, the call will be set up on a GSM network. It will be possible to perform a handover back to a UMTS network when the MS moves into a region where the UMTS network provides coverage. In that way, an existing MS connection will still be maintained. This handover capability is quite important for speech connections, and even more important for on-line data connections.
A basic problem to be resolved with respect to the performance of handovers between GSM and UMTS networks is determining how to transport UMTS measurement information from an MS to the GSM Base Station Controller (BSC). Currently, the GSM (European Telecommunications Standards Institute or ETSI) Standard provides no spare signalling capacity on the up-link (UL) in the active mode. Consequently, the transport of UMTS measurement information from an MS to the GSM BSC will have to be performed at the expense of other information.
A proposed solution to the problem of determining how to transport UMTS measurement information from an MS to a GSM BSC for handovers is to send the measurement information from the UMTS portion of the MS on the GSM Fast Associated Control Channel (FACCH). However, a significant problem with using the FACCH to send UMTS measurement information from an MS to a GSM BSC during a handover is that the FACCH operates in a stealing mode, whereby a 20 ms segment of speech is exchanged for signalling information required for the handover. In other words, during GSM handovers, certain speech frames are used for signalling information instead of for speech information. Consequently, for GSM to UMTS handovers using measurements conveyed on a GSM FACCH, some of the speech frames would be used to convey the UMTS measurement information from the MS to the GSM BSC, which would severely reduce the quality of the speech information being conveyed. However, as described in detail below, the present invention successfully resolves this pressing handover problem and other related problems.
In accordance with a preferred embodiment of the present invention, a method for conveying measurement information from a mobile terminal in a first communication system to a second communication system is provided, whereby the measurement information related to the first communication system is conveyed to the second communication system on a control channel which can function in a non-stealing mode. For example, in accordance with the preferred embodiment, UMTS measurement information can be conveyed from an MS to a GSM BSC, in a GSM message on a Slow Associated Control Channel (SACCH).
An important technical advantage of the present invention is that measurement information can be conveyed effectively between different mobile communication systems without sacrificing the quality of speech information being conveyed.
Another important technical advantage of the present invention is that handovers between different types of mobile communication systems can be effectively performed.
Still another important technical advantage of the present invention is that coverage for a UMTS network can be expanded using GSM network coverage.